A key design parameter for a metal-oxide-semiconductor (MOS) transistor device is the current delivered at a given designed voltage. This parameter is commonly referred to as the drive current or saturation current (IDsat). One factor that has an effect on the drive current is the carrier mobility of the channel region. Increases in the carrier mobility in the channel region result in increases in the drive current. The carriers in NMOS and PMOS transistors are electrons and holes, respectively. The electron mobility of the channel region in NMOS devices may be increased by exposing the region to a uniaxial tensile strain. Alternatively, the hole mobility of the channel region in PMOS devices may be increased by applying a uniaxial compressive strain on the channel region.
Currently, channel strain may be introduced into the device by either epitaxially depositing a layer over the surface of the source/drain (S/D) regions or by replacing the S/D regions with materials that are different than the material used for the channel region. The strain is induced by creating lattice constant mismatches between the layers. For example, when an epitaxially deposited layer is formed over the S/D regions, the epitaxy layer may have a different lattice constant than the S/D regions. Alternatively, when the S/D regions are replaced, the replacement S/D regions may have a lattice constant that is different than the channel region. The amount of strain that can be induced in the channel has several limits. The amount of strain that can be induced in the channel is limited by the physical structure of the transistor. One limiter upon the amount of strain which can be induced in the channel region, is the strain in the substrate material existing beneath the channel region.